Most everyone is familiar with USB, the Universal Serial Bus; it is an incredibly successful technology for providing communication between personal computers and peripherals, and there are millions of nodes in service around the world. USB was originally designed to provide the end user with an extremely simple method for plugging peripherals to a host, such as adding a printer to a PC. USB is now a family of standards managed by the USB Implementers Forum. (See www.usb.org.)

The current core standard is USB 2.0, or Hi-Speed USB. This operates over wires at up to 480 Mbps, but also includes the original USB standard (USB 1.1) for applications where a data rate of 12 Mbps is sufficient (or even the low speed mode of 1.5 Mbps). Additionally, there are standards based on USB 2.0 for wireless use (Wireless USB) and, to cope with needs of portable equipment, USB On-The-Go (technically a supplement to USB 2.0).

Wired USB has a host-based star topology. Arms of the star can also themselves be stars and ultimately they terminate in peripherals. (With USB On-The-Go, devices can act as both hosts and peripherals.) The USB host controller, which is essential for communicating with all USB peripherals, is defined by the USB Host Controller Interface specification, while USB Class Specifications cover selected groups of related USB peripherals. For example the Human Interface Device (HID) Class Specification covers such devices as keyboards, mice, joysticks etc.

Enlarge this picture Fig. 2. Vinculum embedded USB host controller architecture.

Like TCP/IP, USB software is designed as a stack of layers, with the lowest layer providing the interface with the hardware and the highest providing interface to applications or the OS. A USB-Host stack will typically consist of a host controller capability at the lowest level with a middle, services layer handling device connection and disconnection, device initialization, driver selection and resource management, and a top level, containing the different drivers for the different classes of devices, making the device available to the OS and applications.

One particular manifestation of USB — the flash drive — is particularly pervasive. The latest announcement is for drives of 32 gigabytes of data available in a compact but rugged package designed to fit onto a key ring. Four gigabytes are widely available for around $40 retail, while smaller drives, such as 256 megabytes, are currently available for under $10 in low volume. Technology advances will ensure that storage capacity will increase while the cost per gigabyte will decrease year on year for the foreseeable future. This makes USB flash drives the storage medium of choice for sharing files off-line, for back-up and for carrying the inevitable PowerPoint presentation to meetings.

So how is a technology designed for easy plug-and-play of PC peripherals going to be useful in an appliance?

One case study should make this all clear. A supplier of commercial ovens to the fast-food industry has a customer with a rolling program of seasonal special meals, heavily advertised across their sales territories. Normally, staff in the restaurants load prepared meals into the oven and enter a meal code into a control keypad, initiating a sequence of microwave and radiant heat cycles specific to that meal. Whenever the new specials were introduced, the new sequences had to be loaded by a technician, who would use the keypad and its single-line interface for set-up. This is clearly time consuming and inefficient, with a technician traveling to every location.

Enlarge this picture Fig. 3. Circuit schematic shows ease of interface to appliance microcontroller.

There had been experiments with downloading over the Internet, but there were issues of connectivity and cost, particularly for locations with multiple ovens in locations that do not necessarily have existing internet connectivity or would require excessive wiring. Today, the customer is installing new ovens with a built-in USB interface. Instead of the technician visiting, the new heating sequences are loaded from a USB flash drive, which can be easily done by the restaurant manager or a member of staff. (See Fig. 1.)

But the use of a USB flash drive doesn’t just have to be for loading information into an appliance; it can also be used in obtaining information from an appliance. For example, a prototype design can be fitted with sensors for heat, humidity, cycle-counting, and so on. It can then be monitored during testing, with the results downloaded into the flash drive for analysis in a PC later.

USB is designed to be easy to use, but until now, adding a USB host to an application could be difficult, as the USB protocol is complex. To help resolve this issue, Future Technology Devices International (FTDI) developed the Vinculum USB platform, an ASIC that links a USB interface to an MCU using simple and well-understood communications interfaces, such as RS232 or SPI.

Vinculum is derived from a Latin word, meaning to bind or tie. The term was chosen because the FTDI Vinculum device ties together the worlds of USB peripherals, hosts and traditional interfaces. In mathematics, a horizontal bar across two or more terms, showing that they are to be treated together, is also a Vinculum.

Fig. 4. VMUSIC2 module from FTDI.

With the FTDI approach, adding a USB host controller is straightforward, as all the elements needed come in a single-chip solution. To make it even easier, FTDI has produced two snap-in panel-mounting modules containing the Vinculum device, VDRIVE and VMUSIC. Both require only a four-signal pin connection to the control unit of an appliance, and the VMUSIC module includes an MP3 decoder and audio line out.

Within the Vinculum chip are an 8-bit processor, two dedicated DMA (Direct Memory Address) engines, and a 32-bit co-processor to handle the file functions. Popular USB device class stacks are integrated within the Vinculum chip, together with FAT12/16/32 support for USB flash drive applications, thus freeing the product designer from the development and support of these complex and time-consuming layers.

The device is supplied with royalty-free firmware binaries developed in a high-level assembly language and have simple DOS-like high-level commands for USB peripheral control and data transfer. The Vinculum interface to the application MCU can be a traditional UART, SPI or a FIFO. Vinculum can control two USB interfaces complying with Full Speed USB 2.0 (which includes the USB 1.1 and Low Speed USB). Each interface can be configured as either a slave or a host, depending on the firmware chosen.

Vinculum device programming is through the UART interface with upgrades via either UART or USB flash drive. (See Fig. 2.) Supporting the device are development boards, reference designs, including a data-logging module, and the VDRIVE and VMUSIC interface modules. VDRIVE is a simple way to add a USB flash drive interface to an application. It is a snap-in module for panel mounting, with a single USB socket and a traffic indicator LED on the surface. It interfaces through an 8-pin connector, with four signal pins and PWR, GND and wake-up from the low power sleep mode. (See Fig. 3.)

The VMUSIC interface module is a larger snap-in module, with the ability to play back MP3 and other music formats directly from the USB flash drive. In addition to the functionality of the VDRIVE, the VMUSIC module also has line out and a 3.5 mm stereo headset jack socket. (See Fig. 4.)

Since the introduction of Vinculum, FTDI has been working with many different companies to evaluate new application areas for USB flash drives. Most of these are at varying levels of confidentiality, as companies seek to use USB to gain a competitive edge. One interesting area is in the control of HVAC/R equipment.

The USB flash drive can be used, just as in the example of the oven, to load new programming data into the control panel, perhaps to respond to changing use by the system user or to add new equipment. But it can also be used to capture monitoring data for relatively long periods of operations, depending on the capacity of the flash drive, which can be analyzed off-line to improve the performance of the system.

Makers of commercial, coin-operated appliances are also looking at USB flash drives as a tool for upgrading firmware to add new features, to add new product options in dispensing machines, and to upgrade the coin sensor technology in the continual battle with fraudsters. Using a VMUSIC module instead of just a visual interface allows the product to talk an operator through different steps in making changes and upgrades.

Adding USB is a new way to provide product differentiation, both through the USB itself and through the possibility of upgrading to add new features. It is still at early stages, but as developers become more familiar with the technology, new and exciting applications will eventually emerge. FTDI is providing tools that make adding this technology as simple as possible.

For more information, email: sales1@ftdichip.com